1. Field of the Invention
This invention relates to a stimulable phosphor sheet. This invention particularly relates to a stimulable phosphor sheet for use in an energy subtraction processing method for radiation images.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and read-out methods and apparatuses. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of a stimulable phosphor overlaid on the substrate. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. After the image signal has been detected from the stimulable phosphor sheet, the stimulable phosphor sheet is exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet is thereby released.
The image signal, which has been obtained with a radiation image read-out method, or the like, is then subjected to image processing, such as gradation processing or frequency processing, such that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. The image signal, which has been obtained from the image processing, is then reproduced as a visible image on photographic film or on a cathode ray tube (CRT) display device and used in making a diagnosis, or the like. The stimulable phosphor sheet, from which the image signal has been detected, is exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet is released. The erased stimulable phosphor sheet can then be used again for the recording of a radiation image.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, a plurality of (basically, two) radiation images recorded under different conditions are photoelectrically read out, and digital image signals, which represent the radiation images, are thereby obtained. The image signal components of the digital image signals, which image signal components represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, the plurality of digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure, which has radiation energy absorption characteristics different from the radiation energy absorption characteristics of the other parts of the object, can be reproduced from the difference signal.
With regard to the subtraction processing, the applicant proposed a stimulable phosphor sheet comprising a substrate, which is constituted of a substance capable of absorbing low energy components of radiation (an energy distribution separating layer), and stimulable phosphor layers, which are overlaid respectively on the two surfaces of the substrate. Such a stimulable phosphor sheet is disclosed in, for example, U.S. Pat. No. 4,855,598. In cases where the proposed stimulable phosphor sheet is exposed to radiation carrying image information of an object, radiation images of the object are stored respectively on the two stimulable phosphor layers. On the stimulable phosphor layer, which is located on the side opposite to the object side, a radiation image is stored with the radiation, in which the low energy components have been reduced to an amount smaller than the amount of the low energy components contained in the radiation impinging upon the stimulable phosphor layer located on the object side. Stimulating rays are then irradiated to the two surfaces of the stimulable phosphor sheet or to only either one of the two surfaces of the stimulable phosphor sheet. Light emitted by one of the two surfaces of the stimulable phosphor sheet and light emitted by the other surface of the stimulable phosphor sheet are photoelectrically detected, and two image signals are thereby obtained. The subtraction process is then carried out on the two image signals. In this manner, a subtraction image can be efficiently obtained with a single image recording operation.
In cases where the stimulating rays are thus irradiated to only either one surface of the stimulable phosphor sheet and the two image signals are respectively obtained from the two stimulable phosphor layers overlaid on opposite sides of the substrate, it is necessary that the stimulating rays also stimulate the stimulable phosphor layer, which is located on the other surface of the stimulable phosphor sheet. Therefore, it is necessary for the substrate to be constituted of a material, which is capable of transmitting the stimulating rays. Such a requirement is described in, for example, Japanese Unexamined Patent Publication No. 6(1994)-130197.
In cases where the light emitted by one of the two surfaces of the stimulable phosphor sheet and the light emitted by the other surface of the stimulable phosphor sheet are detected respectively, two image signals being thereby obtained, and a subtraction image is obtained from the two image signals, the problems described below occur. Specifically, when an image signal is detected on either surface side of the stimulable phosphor sheet, the light, which has been emitted by the stimulable phosphor layer located on the opposite surface side of the stimulable phosphor sheet, passes through the substrate towards the surface side of the stimulable phosphor sheet, on which side the detection of the image signal is being carried out. Therefore, the light, which has been emitted by the stimulable phosphor layer located on the opposite surface side of the stimulable phosphor sheet, is mixed into the light emitted by the stimulable phosphor layer located on the surface side of the stimulable phosphor sheet, on which side the detection of the image signal is being carried out. As a result, the problems occur in that the radiation image, which has been stored on the stimulable phosphor layer located on one of the two surface sides of the stimulable phosphor sheet, and the radiation image, which has been stored on the stimulable phosphor layer located on the other surface side of the stimulable phosphor sheet, cannot be accurately separated from each other, and a subtraction image having a high signal-to-noise ratio cannot be obtained.